Project Summary Cryptococcosis is an opportunistic infection that occurs in immunocompromised individuals including AIDS patients, organ transplant recipients, cancer patients, and those treated with immunosuppressive therapies. It is estimated there are one million cases of cryptococcosis and proximately 600,000 deaths worldwide annually. The infection starts in the lung via inhalation of the encapsulated fungus Cryptococcus neoformans. However, the major and most lethal complication of cryptococcosis is meningoencephalitis. To cause meningoencephalitis, C. neoformans must disseminate from the lung and enter the bloodstream. As such, fungemia, detected frequently in AIDS patients during cryptococcosis, is believed to be one of the most critical steps in the development and persistence of cryptococcal meningoencephalitis. However, little is known about intravascular clearance of the disseminating C. neoformans due to technical challenge for in vivo studies. In particular, a critical gap in our understanding remains: Does a mechanism exist to actively eliminate disseminating C. neoformans out of vasculature, and what is the mechanism? Liver is situated at a confluence of arterial and venous blood. Kupffer cells (KCs) constitute ~90% of the tissue macrophages in the whole body and account for ~15% of the total liver cell population. CRIg, a unique complement receptor, has been recently identified on KCs of humans and animals. Although the liver is not a target organ of cryptococcosis, we hypothesize, based on our preliminary data, that KCs play a critical role in filtering of disseminating C. neoformans out of circulation via a mechanism involving complement receptor CRIg. We will test the hypothesis by addressing the following aims using intravital microscopy and other advanced approaches: (1) To characterize the role of KCs in filtering C. neoformans out of vasculature; (2) To dissect the mechanism(s) involved in capture of disseminating C. neoformans by KCs. The findings from this study will suggest that a therapeutic strategy aimed at enhancing macrophage recruitment and activity in the liver could help reducing the risk of cryptococcal meningocephalitis in AIDS patients.